An alternative pathway of leucine catabolism involves the action of the enzymes leucine mutase, Beta-leucine-alpha-ketoglutarate transaminase, and beta-ketoisocaproate thiolase. Leucine mutase will be purified and characterized and its B12 involvement will be established. The remaining enzymes will be examined and the co-factors will be established. The distribution of the pathway in nature will be explored and its importance in human metabolism will be examined.